Magnetic droplets are nontopological dynamical solitons that can be nucleated in nanocontact based spin torque nano-oscillators (STNOs) with perpendicular magnetic anisotropy free layers. While theory predicts that the droplet should be of the same size as the nanocontact, its inherent drift instability has thwarted attempts at observing it directly using microscopy techniques. Here, we demonstrate highly stable magnetic droplets in all-perpendicular STNOs and present the first detailed droplet images using scanning transmission X-ray microscopy. In contrast to theoretical predictions, we find that the droplet diameter is about twice as large as the nanocontact. By extending the original droplet theory to properly account for the lateral current spread underneath the nanocontact, we show that the large discrepancy primarily arises from current-in-plane Zhang-Li torque adding an outward pressure on the droplet perimeter. Electrical measurements on droplets nucleated using a reversed current in the antiparallel state corroborate this picture.

Several scientific issues concerning the latest generation read heads for magnetic storage devices, based on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) are known to be controversial, including such fundamental questions as to the behavior and the role of B in optimizing the physical properties of these devices. Quantitatively establishing the internal structures of several such devices with different annealing conditions using hard x-ray photoelectron spectroscopy, we resolve these controversies and establish that the B diffusion is controlled by the capping Ta layer, though Ta is physically separated from the layer with B by several nanometers. While explaining this unusual phenomenon, we also provide insight into why the tunneling magnetoresistance (TMR) is optimized at an intermediate annealing temperature, relating it to B diffusion, coupled with our studies based on x-ray diffraction and magnetic studies.

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We report on the magnetization depth profile of a hybrid exchange spring system in which a Co/Pd multilayer with perpendicular anisotropy is coupled to a CoFeB thin film with in-plane anisotropy. The competition between these two orthogonal anisotropies promotes a strong depth dependence of the magnetization orientation. The angle of the magnetization vector is sensitive both to the strength of the individual anisotropies and to the local exchange constant, and is thus tunable by changing the thickness of the CoFeB layer and by substituting Ni for Pd in one layer of the Co/Pd stack. The resulting magnetic depth profiles are directly probed by element specific x-ray magnetic circular dichroism (XMCD) of the Co, Fe, and Ni layers located at different average depths. The experimental results are corroborated by micromagnetic simulations.PACS numbers: 75.30. Gw, 75.30.Et, 78.20.Ls, 75.70.Cn a Author to whom correspondence should be addressed. Electronic mail: anhntn@kth.se. 2The phenomenon of spin transfer torque (STT) [1][2][3], in which a spin-polarized current transfers angular momentum to a magnetic layer, has brought about novel applications such as spin torque oscillators (STOs) [4][5][6][7] and spin transfer torque magnetoresistive random access memory (STT-MRAM) [8][9][10]. While initially based on magnetic materials with in-plane (IP) magnetic anisotropy, the realization that such materials lead to unnecessarily high STT-MRAM switching currents, poor memory retention, poor scalability [11], and high-field operation of STOs [12], there is now a rapidly growing interest in fabricating STT devices based on perpendicular magnetic anisotropy (PMA) materials. Recent tailoring of PMA materials and their interfaces have demonstrated low switching currents, high switching speed, good thermal stability, future scalability [9,[13][14][15], and low-to zero-field operation of STOs [16][17][18][19][20].Building upon these successes, the natural extension of using PMA materials is to also investigate the potential of devices in which the magnetization is tilted with respect to the surface normal. Such materials allow for additional control of the magnetization dynamics in magnetic nanostructures [17,[21][22][23], and hint at yet improved STT-MRAM switching behavior and thermal stability [24][25][26][27][28][29]. For STOs, tilted materials offer a route to improve their microwave generation properties, both in terms of higher output power and low-to zero-field operation [17,[21][22][23]28,[30][31][32].Recently, tilted materials have also been shown to have potential for current-driven domain wall motion [33]. The influence of a tilted anisotropy is stronger than simply tilting the applied field [34] as a mere 5 degree misalignment between the free and the fixed layer in magnetic tunnel junctions (MTJs) can reduce the switching current by 36%, the switching time by 30%, and improve the switching current distribution [35].Materials with tilted anisotropies have been realized using collimated oblique sputtering [36], depos...